Eating a variety of plants will help compliment proteins into adequate levels. Even a tomato has a gram of protein (mostly glutamic acid but that converts to glutamine pretty easily). The trick is consistent emphasis on staples like beans, rice, grains, and variety. Try new combinations of legumes, peas, and exotic grains as well as myco (fungi)-proteins such as the brand Quorn. Easy to gain or maintain muscle with variety and consistency.